Thursday, April 9, 2009

Schematic description
Wattmeter schematic explained

The schematics for Wattmeter is based on the PIC 16F876 device. The PIC microprocessor has a crystal of 4MHz in order to have a true calculation power of 1 MIPS. There are four other parts, the power supply, the RS232 interface, the input signal conditioning and the display driver. 
The wattmeter is equipped by a standard 9 Volt battery and the power supply for PIC and the other components is a linear regulator LM78L05, capable to source 100mA at 5V. The PIC interfaces the PC serial port using a MAX232 that translate the TTL levels to true RS232 levels. The version of the MAX232 used allow the usage of 100N capacitors for charge pumps. The RS232 connection is used for future expansions (PC remote control and monitor, boot loader). 
The input voltage is measured by the PIC, via two simple resistors and a MC33201 operational amplifier, that gives a measurement range from 0 to 19 V. The current transducer is the LEM LTS 25 NP with a range of ±25A (the usable range is about 3 times this limit). Another MC33201, with the same configuration used for voltage measurement, is used for current/voltage conversion. 
The display driver is an ICM7211 with 4 digits and 3 position for the dot. The display driver requires a BCD input and a digit Strobe. 
The display can shows the power (P=V*A) in the form PPP.P watt (resolution 0.1W), the voltage in the form TT.TT volt (resolution 10mV), or the current in the form ±CC.C ampere (resolution 0.1A). The operator can choose one of the previous measure by a three position switch. The central point of two resistance connected in serial mode (the other terminals are connected to +5V and GND respectively) is joined to an analog Pic input. The switch can shunt one of the two resistance (or no one) obtaining a +5V, +2.5 V or 0V input for the AN2 Pic input.

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